The present invention relates to an electrophotographic recording material comprising an electrically conductive support, an optional insulating barrier layer, and a photoconductive double layer comprising a layer which generates charge carriers and a charge transport layer, which double layer contains an organic photoconductor, a binder, a dye, and, optionally, conventional additives. The present invention further relates to a process for the manufacture of the aforementioned electrophotographic recording material.
The use in electrophotographic recording materials of photoconductive layers which are composed of a photoconductor and a material acting as a binder is disclosed in U.S. Pat. No. 3,121,006. Binder layers are described which contain finely divided particles of a photoconductive inorganic compound, dispersed in an electrically insulating organic binder. The binder is a material which is incapable of transporting charge carriers generated by the photoconductor particles over a significant distance. Consequently, the photoconductive pigment particles within the layer must be in virtually continuous contact to enable the charges to be conducted away. Conductivity or charge transport is provided by a high concentration of the photoconductive pigment. With such a layer structure, a pigment concentration of more than 50 percent by weight is required.
It has also been disclosed (German Offenlegungsschrift No. 2,108,992, corresponding to U.S. Pat. No. 3,904,407) to produce photoconductive layers for electrophotographic recording materials in a double-layer arrangement. The disclosed material is composed of an electrically conductive support, a layer generating charge carriers, and a charge transport layer. The layer generating charge carriers can then comprise a dispersed pigment. If an insulating binder is used together with the dispersed pigment, a volume concentration of at least 25% of pigment is necessary. The ratio of the layer thicknesses of the charge transport layer and the layer generating charge carriers is 2:1 to 200:1.
It has also been disclosed (German Offenlegungsschrift No. 2,160,812, corresponding to U.S. Pat. No. 4,026,704) to provide photoconductive layers comprising a top layer and bottom layer, both of which contain a binder and the same organic photoconductor, the bottom layer additionally containing at least one activating sensitizer in a quantity of 1 to 20 percent by weight, relative to the total photoconductor content. The disclosed top layer comprises a binder and up to 50% by weight of photoconductor. The indicated layer thicknesses are 0.1 to 5 .mu.m for the bottom layer and 5 to 20 .mu.m for the top layer.
In order to improve the resolution of images obtained by an electrophotographic method, it has also been disclosed (U.S. Pat. No. 3,533,783) to use photoconductive layers which, in a bottom layer, contain an inorganic or organic photoconductor together with an activator, such as a pyrylium salt, and contain a photoconductor and binder in the top layer. The indicated thicknesses of the layers are generally 2.5 to 25 .mu.m.
German Offenlegungsschrift No. 3,108,618 (corresponding to U.S. Pat. No. 4,340,658) describes a three-layer photoreceptor in which a pigment concentration of 50 to 95 percent by weight in the binder employed is necessary.
It has also been disclosed (German Pat. No. 1,117,391, corresponding to British Pat. No. 944,126) to use photoconductive, low-molecular organic compounds for the manufacture of printing plates by an electrophotographic method and to sensitize these compounds in the visible region of the spectrum by means of suitable dissolved dyes (corresponding to German Offenlegungsschrift No. 2,526,720, equivalent to U.S. Pat. No. 4,063,948). In place of the low-molecular substances, polymeric photoconductors together with an activator can also be used (German Offenlegungsschrift No. 2,726,116).
A disadvantage of the known electrophotographic recording materials with a binder, organic photoconductor and dye or pigment is their relatively unsatisfactory resolution, which manifests itself especially when a latent image charged at negative polarity is developed with a liquid developer. Individual lines of a line width below 60 .mu.m then form an image of only reduced contrast, and lines of under 40 .mu.m line width form no image at all. These resolution losses also occur in the case of correspondingly fine screen dots. A further disadvantage is the relatively high content of photoconductor. Thus, in order to obtain an adequate light sensitivity, the photoconductive layers must contain the organic photoconductor in a total concentration of 40 to 50 percent by weight, in addition to the insulating binder, and this manifests itself in considerably increased costs of the materials.
In the manufacture of printing forms by an electrophotographic method, it is also important that the organic photoconductors are insoluble in aqueous-alkaline decoating solutions. Decoating, such as is necessary in the case of use for printing plates and printed circuits, is therefore also impeded by these components. Moreover, the insoluble fractions deposit on rolls, pumps and other components in the decoating apparatus and cause increased maintenance costs.
Since, in the known double-layer materials, the transport layer with its large fraction of photoconductor is also thicker than the layer which generates charge carriers, the disadvantages described above also occur in such materials. Photoconductor double layers, for example, according to German Offenlegungsschrift No. 2,108,992, which are applied in 4 .mu.m thickness to an aluminum support, give charge capacities which are inadequate for use in practice. Satisfactory results are obtained only with layer weights of more than 10 g/m.sup.2, with considerable expense of material. A final disadvantage is that dye or pigment particles from the large fraction in the layer generating charge carriers, since they are in contact with the metallic substrate which is the support, become embedded in the pores of the surface, from which they are not removable later. Printing plates produced in this way scum on printing and are virtually useless.